Internal combustion engines typically utilize an exhaust gas recirculation (EGR) system to recirculate a controlled portion of exhaust gas generated by the engine into an intake manifold of the engine in order to provide a reduction in NO.sub.x emissions generated by the engine. Typically, a control mechanism is provided which varies the EGR rate according to one or more sensed conditions such as engine temperature, air charge entering the intake manifold of the engine, and engine speed. Some systems such as described by Romblom et al. in U.S. Pat. No. 4,399,799 also adjust the EGR rate as a function of barometric pressure to compensate for differences in air pressure which exist at different altitudes. While Romblom et al. describe a mechanical EGR control system, other systems utilize an electronic controller to determine EGR rate as a function of inferred barometric pressure.
The inventors herein have recognized that such systems, while providing improved performance and reduced emissions, exhibit sensitivity to inferred barometric pressure, which is prone to error, and also fail to provide an optimum EGR rate over a variety of altitudes.
Accordingly, there is a need for a system which provides an optimum EGR rate at a variety of engine operating ranges and atmospheric conditions without the extensive calibration and cost required in known systems.